Organic electronic devices

ABSTRACT

A method for forming an organic electronic device, which method comprises the steps of:
         a) forming a negative image of a desired pattern on a substrate or device layer with a lift-off ink;   b) coating a first device layer to be patterned on top of the negative image;   c) coating one or more further device layers to be patterned on top of the first device layer to be patterned; and   d) removing the lift-off ink and unwanted portions of the device layers above it, thereby leaving the desired pattern of device layers.       

     The method allows the formation of a device structure wherein the device layers to be patterned are self-aligned. The method enables a multiplicity of layers to be patterned in a single set of printing and lift-off steps using one pattern which ensures the excellent vertical alignment of edges, which would be difficult to achieve by direct printing. Horizontal alignment can also be achieved. The size of the device features can be reduced below the actual printing resolution. Examples of organic electronic devices include OFETs, OLEDs, memory, sensing elements, solar cells, photo-sensors, photoreceptors for electrophotography and the like.

This invention relates to organic electronic devices, to a process fortheir manufacture and to their uses, particularly but not exclusively toimproved techniques that are especially suitable for the low costfabrication of devices and circuits made from solution coatable organicmaterials by printing. Examples of organic electronic devices includeorganic field effect transistors (OFET), organic light emitting diodes(OLED), memory, sensing elements and the like. Such devices can beassembled into circuits, display devices, radio-frequency tags and thelike.

Conventional electronic devices are based on inorganic semiconductorssuch as Si or GaAs. However, inorganic devices are often difficult andexpensive to manufacture due to the high temperature processingconditions and the vacuum equipment required, particularly whenfabricating large area devices e.g. display driver circuitry currentlymanufactured from amorphous or polycrystalline silicon.

Organic electronic devices offer the advantage of low temperaturemanufacturing on large areas, under ambient conditions and usingflexible substrates. The possibility to use solution depositiontechniques, especially printing, is very attractive. The term organicelectronic device used herein means any electronic device having atleast one layer which contains organic material, e.g. an organicsemiconductor (OSC). Examples of organic electronic devices includeOFETs and OLEDs.

Various techniques have been used for the fabrication of organiccircuitry based on conjugated polymers. A recent review (S. Holdcroft inAdvanced Materials, 2001, No23. p1753) describes these techniques.Techniques include conventional lithography; electron beam lithography;advanced techniques, such as scanning probe microscopy (SPM); directphotopatterning; photochemical patterning; photolithography; directprinting techniques, such as screen printing; micromoulding incapillaries (MIMIC); microcontact printing (or “soft lithography”); andnon-impact printing techniques, such as ink-jet printing. Each of thesetechniques suffers one or more disadvantage.

Of the techniques above for patterning flexible organic circuits,photolithography offers the highest resolution, but, along withphotochemical patterning, large area patterning is difficult to performwith good alignment and expensive particularly when specially madephotomasks are required for each separate layer of a multilayer device.SPM is only feasible on very small substrates. Direct photopatterning,requires the complete conversion of an active material, for example,from a conductive to a nonconductive state, full conversion is difficultto achieve and reactive sites may remain.

Direct printing techniques can achieve resolutions of 100 μm, but theyrequire accurate physical contact with the mask or screen to stop inkflowing onto unwanted areas. Further, the material to be printed has tobe formulated into an ink with the right viscosity and wettingproperties, this is difficult to achieve because many conjugatedpolymers which would otherwise be suitable for OFETs have undesirablerheological properties. MIMIC uses a mould for example of a preformedelastomer, such as a poly(dimethylsiloxane) (PMDS). This mould is placedon a substrate to be patterned and a liquid formulation is placed sothat it can be drawn by capillary action into recessed channels of themould. After curing or drying the ink the mould can be removed. OFETchannels of 25 μm length can be fabricated by this technique, however,the patterned areas must be connected to each other via channels and itis difficult to prevent trapping bubbles within the fine features of acomplex channel pattern. A further disadvantage is where severalmaterials need to be patterned when it is difficult to align severallayers accurately enough as the process is repeated. Microcontactprinting relies on an active layer being formulated into an ink, andwhen used for the deposition of an etch mask, the layer to be patternedmust be soluble or etchable. In non-impact printing techniques, such asink-jet, it is not easy to formulate active materials of the circuitinto an ink which can be ink-jetted, and direct deposition of thematerial leads to deposition of a non-uniform layer pattern.

A number of methods are disclosed in the literature for the manufactureof OLED displays. A recent review can be found in: C.R. Acad. Sci., SerIV: Phys., Astrophys. (2000), 1(4), 493-508.

For manufacturing OLEDs from solution processable materials spin coatingis routinely employed to give uniform thin films. The patterning ofthese devices can be achieved by forming a 3D-pixellated insulatingstructure (wells) on the injecting electrode by photolithography.However, deposition of solutions of electroluminescent materials inthese wells can be problematic since the liquid tends to wet the wallsof the well and this results in an uneven layer. Light emission anddevice efficiency of OLEDs is sensitive to this layer thickness,therefore photolithography is not entirely suitable for patterning OLEDdevices.

Screen-printing has been used to pattern either hole transport materialsor emissive polymers in the manufacture of OLEDs. In each case ascreen-printing fluid of an active material is prepared and patternedthrough a preformed mesh using a squeegee (Advanced Materials (2000),12(17), 1249-1251). This method is not entirely suitable becauseproblems are encountered in obtaining uniform film thickness, transferof the mesh pattern and feature size resolution. Furthermore, thematerial to be patterned needs to be of the correct viscosity for screenprinting, which is not always possible.

Ink-jet printing of OLED materials has been reported. For example,deposition of Nile Red doped into poly-vinylcarbazole onto a flexiblesubstrate has been reported (Appl. Phys. Letts, 72(5) 519-521). However,uneven polymer distribution within the drops, domed topography of thedrops, pin-holing and uneven light emission are reported problems.

The fabrication of OLED devices using the ink-jet deposition of anactive material(s) (whether OLED, hole or electron transport material orconductive layer such as polyaniline (PANI) orpoly(3,4-ethylenedioxy)thiophene-2,5-diyl (PEDOT) has been reportedextensively. In all cases the active material is printed into apreformed pixellated substrate prepared by standard lithography (e.g. JP10012377 (SEC); WO9907189 (Cambridge Consultants); WO9912397 (SEC);WO9939373 (Sturm)). Again, as with direct printing techniques generally,the materials need to be formulated into an active ink, which is notalways possible.

Of the above techniques those which take advantage of liquid coatabilityof organic materials are much preferred. It is especially desirable thatnon-reactive techniques are used due to the sensitivity of OLEDmaterials to the strong UV light or chemical photoinitiators used inphotolithography. Printing techniques, e.g. inkjet, screen printing orsoft lithography (Michel et al, IBM J. Res. & Dev., Vol 45, p697, 2001)hold much promise as the achievable resolution increases for example, byusing novel ink-jet head and stamp materials. However, printing by thesetechniques requires the formulation of an active layer e.g.electroluminescent material into an effective ink. This can beproblematic since the active materials are often only soluble inaggressive organic solvents or in acidic aqueous media. These can giverise to material compatibility issues either with the ink-jet head orwith the stamp material. Accurate printing requires the adjustment ofmany parameters such as viscosity, levelling flow and drying propertiesof the formulation. This is not always easily achievable and solubleOLED materials are also sensitive to additives used in printingprocesses. Thus there is a need for improved techniques that still relyon printing, for example ink-jet, screen printing or microcontactprinting, but resolve some of the issues of formulating OLED materialsinto inks.

Methods of “indirect” printing can solve the problem of formulating thematerials into inks. EP0193820 (Kanegafuchi) describes such a methodwhich includes forming a thin film pattern by forming a “lift-off layer”onto which the film to be patterned is deposited. The “lift-off layer”can be created by screen or ink jet printing. The method is describedfor use to pattern metal, silicide, amorphous or crystallinesemiconductor and insulator. Possible applications include solar cells,photo sensors, photoreceptors for electrophotography, thin film diodes,and transistors. Examples provided are amorphous silicon solar cells.The invention does not teach or suggest the manufacture of OFET circuitsnor OLEDs or displays. It discloses the patterning of vacuum depositedlayers and does not relate to the patterning of solution coatableorganic semi-conductor (OSC) materials. Furthermore and importantly, themethod provided limits the achievable feature sizes to the resolution ofthe printing technique. Moreover, layer registration or alignmentproblems are not resolved by the technique.

WO 01/17041A1 (E-Ink) describes a technique for patterning where asemiconductor is affected locally by a “destructive agent”. Thedestructive agent may be printed by inkjet, screen printing and thelike, thus creating a patterned change in the semiconductor. Thisdocument also describes the use of a “release” layer to pattern anorganic semiconductor layer. However, registration or alignment problemsremain and the device features are limited by the printing resolution.

In summary, as indicated above, a number of approaches for direct orindirect printing of organic devices have been published. Directprinting techniques rely on formulating an electronically activematerial e.g. a semiconductor into a printing ink. Unfortunately, notall elements of organic devices are readily formulated into a printableink. Accurate direct printing of an OSC requires the adjustment of manyparameters such as viscosity, levelling, flow and drying properties ofthe formulation. This is not always easily achievable and solublesemiconductor materials are sensitive to additives used in printingprocesses. Thus there is a need for improved techniques that still relyon printing but resolve some of the issues of formulating OSC materialsinto inks. Some indirect methods resolve this problem, however, printresolution is still limited. There is a need to pattern layers oforganic devices by ambient printing processes and a need for printingprocesses which resolve registration issues when one layer is printedabove another and offer high resolution at low cost.

We have surprisingly discovered that using indirect patterning byprinting there is a possibility to reduce device features below theactual print resolution. It is also possible to provide for very highdegree of registration and alignment between layers despite the limitedresolution of the actual print process. Using a printing process toachieve this is particularly advantageous for large area organicdevices.

The present invention provides a method of forming an organic electronicdevice by printing. Advantageously, working features can be made smallerthan the resolution of the printing method used. High accuracyself-alignment and registration is achieved between two or more layersby using a relatively low resolution printing technique.

According to the present invention, in a first aspect, there is aprovided a method for forming an organic electronic device, which methodcomprises the steps of:

-   -   a) forming a negative image of a desired pattern on a substrate        or layer of the device with a lift-off ink;    -   b) coating a first device layer to be patterned on top of the        negative image;    -   c) coating one or more further device layers to be patterned on        top of the first device layer to be patterned; and    -   d) removing the lift-off ink and unwanted portions of the device        layers above it, thereby leaving the desired pattern of device        layers.

According to the present invention, in another aspect, there is providedan organic electronic device obtainable by the method of the firstaspect.

The method advantageously allows the formation of a device structurewherein the device layers to be patterned are self-aligned. The presentinvention enables a multiplicity of layers to be patterned in a singleset of printing and lift-off steps using one pattern which ensures theexcellent alignment of edges, which would be difficult to achieve bydirect printing.

Examples of organic electronic devices include OFETs, OLEDs, memory,sensing elements, solar cells, photo-sensors, photoreceptors forelectrophotography and the like.

Further advantages of the present method are that it:

-   -   provides a simple, cost effective way to manufacture organic        electronic devices;    -   provides a simple, cost effective way of reducing feature sizes        even with low resolution print techniques;    -   removes the need for formulating organic materials into inks;    -   avoids the need to use additives;    -   avoids registration problems by providing a highly accurate        means of printing one layer on top of another; and    -   allows the creation of new multilayer structures    -   provides a means of obtaining good device layer thickness        uniformity;    -   provides a way to print vertical field effect transistors;    -   provides a technique to pattern multicolour small molecule        OLEDs; and    -   avoids formation of pre-defined wells used in prior art OLED        manufacture.

These advantages are explained in more detail below.

The present method eliminates several problems encountered with themanufacture of organic electronic devices, for example,

-   -   It is possible to overcome the issue of formulating OSC,        insulator and conductor materials into inks as would be needed        for direct printing. Thus, there is no need to compromise the        materials by the use of additives or unwanted solvents. The ink        can be chosen which is best suited to achieve high resolution        and the same ink and printhead can be used for defining a        variety of layers.    -   The deposition technique for the layer materials(s) may remain        solution coating, but the choice of the techniques is wide        because resolution is not now defined by this step.    -   The thickness uniformity of the final pattern is excellent,        especially at the edges, which is particularly important for        thin layers. This is difficult to achieve by direct printing.        The use of lithographically patterned wells, with their inherent        problems, may be avoided.    -   The method provides a means to achieve high accuracy of        alignment between layers by simple and cheap printing        techniques. The printed pattern can be used to affect lift-off        of several layers at the same time. As a result, these layers        will ‘self align’ in a highly effective manner. No direct        printing technique has so far achieved such well-aligned overlay        of vertical layers. There has been a previously unfulfilled need        to print OLEDs and OFETs at high resolution, for example        reducing gate to drain overlap which has not been solved in any        printed OFET.    -   The use of self-alignment by the combination of print and        lift-off makes it possible to achieve new OFET structures, such        as vertical OFETs, as well as new OLED structures, e.g.        multilayer OLEDs or red, green and blue pixels in good        alignment, by low cost printing.        2. In one embodiment, the invention provides a method of        patterning OFET circuits generally by relatively simple printing        techniques ensuring that, if desired, all manufacturing steps        may remain based on solution or liquid coating processes, which        can be operated at under ambient conditions. The method can        integrate the patterning of semiconductors, insulators,        conductors and dopants for solution coated OFETs and integrated        circuits using the same printing processes even if their direct        patterning is difficult. The method can be fully ambient, and if        required all manufacturing steps may remain based on solution        coating. The OFET circuits may be used to manufacture electronic        devices such as radio frequency tags, display drivers,        oscillators, logic circuitry, sensor circuitry and the like. The        invention provides a method of forming a vertical OFET. The        invention also provides a method of forming an organic        electronic device wherein the step d) forms one or more via (or        interconnect) openings. The via openings may then be filled, for        example by a conducting material.

In another embodiment, the method of the present invention can integratethe patterning of organic light emitting materials (polymer or smallmolecule), blocking layers, injection or transport layers, cathode andanode materials, display pixel interconnect layers and dopants fororganics OLEDs and OLED displays. Again, the process can be fullyambient, and if required, all manufacturing steps can remain based onsolution coating. The OLED devices may form part of an active or passivedisplay matrix. The method can also be used as a complementary tooltogether with direct patterning techniques for OLEDs, such as thosedescribed in the art. The OLED elements may be used to manufacturemulticolour displays, small or large area signs, logos or illuminatingfeatures.

The negative image of the desired pattern is preferably formed using alift-off ink. The ink is printed onto the substrate or layer of thedevice. The term ink herein means a substance capable of being printed,rather than meaning that it must contain a colorant. The lift-off inkcan be any substance which can be patterned and then removed by alift-off step. The lift-off ink is preferably insoluble in the medium ormedia used to deposit the layers to be patterned. The lift-off inkmedium is preferably a liquid which does not dissolve the substrate orlayer on which the lift-off ink is printed. The lift-off ink medium canbe either aqueous or non aqueous. For example, a water based lift-offink is suitable on polymer surfaces that are insoluble in water, e.g.polyesters. An advantage is that the lift-off ink does not necessarilyneed to be electronic grade as it is removed in the process togetherwith the layer to be patterned. The lift-off ink may be flexible incomposition and include flow and other additives. When used withscreen-printing applications, the lift-off ink can have a very highviscosity, up to 90,000 cp, preferably up to 70,000 cp, and morepreferably between 500 and 10,000 cp. But when used with ink-jetprinting, the ink viscosity is preferably in the range from about 0.7 to100 cp, and more preferably from about 3 to 40 cp. The ink preferablyhas a surface tension from 20 to 70 dynes/cm, more preferably 20 to 60dynes/cm. This will be governed both by the mode of printing, choice ofinkjet printing head and the surface energy of the surface to beprinted. Since good edge acuity is required then the surface tension ofthe lift-off ink relative to the substrate is preferably from 20 to 110deg and more preferably 40 to 80 deg. For an inkjet lift-off ink thecontact angle with the nozzle plate is preferably from 10 to 150 deg.

The lift-off ink may be in a liquid medium that may be polar ornon-polar. The liquid medium preferably has a boiling point in the rangefrom 40° C. to 300° C. Preferred liquid media include, but are notlimited to, water; alcohols such as methanol and ethanol; dioxane,aromatic hydrocarbons such as toluene and xylene; haloalkanes such aschloroform and 1,2-dichloroethane; ethers such as tetrahydrofuran,haloarenes such as dichlorobenzene; glycols, and cyclic amides. Thelift-off ink preferably contains from 50% to 99.8% liquid medium, byweight. Liquid medium mixtures are preferred to help control inkapplication properties such as latency, substrate wetting and dryingtime.

The lift-off ink may further comprise a colorant, a polymeric binder andfunctional additives, which are used to modify the ink viscosity,surface tension and latency. Suitable colorants for the lift offcomposition include dyes or pigments, such as carbon black. Suitablepolymeric binders for the lift-off ink include, but are not limited to,acrylics, polyurethanes or silanes.

Cross-linking agents can be included in the lift-off ink to permitcross-linking of the printed ink. This modifies the lift-off parameterseither through partial shrinkage to aid lift-off or to improveresistance to the subsequent coating solution. Cross-linking agents arepreferably added to the ink in a concentration in a range from 0.5 to 30wt. % of the solid ingredients, and more preferably from 1 to 10 wt. %of the solid ingredients. Partial shrinkage or micro-cracks may beinduced, for example, by heat or light curing. This way the efficiencyof the lift-off step may be improved by allowing the lift-off medium topenetrate the ink at the pattern edges or through its surface.

The wetting of the ink formulation may be optimised by the surfacetreatment of the substrate, for example, by plasma treatment. Suchtreatment may also be used to enhance adhesion of the layer to bepatterned to the substrate or improve edge acuity. As a result, thelift-off of the lift-off ink together with the layer above may be moreefficient. The technique can be further optimised by using intermediatelayers coated between the ink pattern and the layer to be patterned.Such layers can be used as barriers stopping the diffusion of ink intoother layers.

The lift-off ink may be deposited on the substrate or layer of thedevice by a direct printing technique. Suitable direct printingtechniques include ink-jet printing, screen printing, microcontactprinting, stamping, soft lithography or electrophotographic printingusing a liquid or solid toner. Ink-jet printing is particularlypreferred. The term ink thus includes toner. In each case the ink isformulated to the appropriate viscosity, rheology and surface tensionfor the specific printing process. The use of ink-jet printing isadvantageous because the same ink formulation and ink-jet head may beused, followed by the same chemical or mechanical process for thelift-off step, for the patterning of different layers therebysimplifying the hardware required. The printed lift-off ink ispreferably thicker than the layer subsequently deposited onto it, thisimproves the efficiency of the lift-off step. The lift-off pattern ispreferably from 100 nm to 100 μm thick, more preferably from 1 μm to 50μm.

The ink may optionally be used in one or more additional steps as anetch mask.

The device layers to be patterned may each independently be applied by avariety of coating and printing techniques. Examples include spin-,spray-, dip-, web-, die- or evaporation coating; electroless depositionand ink-jet printing, screen printing, microcontact printing, stampingor soft lithography. When OLED layers to be patterned are deposited byink-jet printing, selective deposition on different areas is possible.For example, red, green and blue electroluminescent materials may bedeposited on different areas. When OFET layers to be patterned aredeposited by ink-jet printing, selective deposition on different areasis possible. For example, n or p type organic materials may be depositedon different areas. Subsequently the pattern is defined by theunderlying lift-off ink, offering better resolution since more than onematerial is patterned by the same lift-off layer deposited in a singleprinting step.

The thickness of the device layer or multiplicity of layers may be from1 nm (in case of a monolayer) to 10 μm, preferably from 1 nm to 1 μm,more preferably from 1 nm to 500 nm. The preferred deposition techniquefor the device layers to be patterned is a liquid coating technique,more preferably spin-, die- or spray-coating.

Once the lift-off ink is printed and the device layers to be patternedare deposited above it, the step of lift-off can be carried out bydissolving the lift-off ink using a liquid medium. During this step thelift-off pattern is removed together with parts of the device layers tobe patterned. Any liquid medium may be employed for this, as long as itdissolves little or none of the device layers to be patterned which areon the substrate, or in multilayer devices on an earlier patternedlayer. Preferred liquid media include water, alcohols such as methanoland ethanol. Liquid media may be used alone or in combination with otherliquid media. The efficiency of the lift-off part of the process may beenhanced by ultrasonic agitation, stirring, spraying liquid mediumand/or heating. The lift-off part of the process may be optionallyeffected by abrasion, high pressure air or other mechanical action.

Various substrates may be used for the fabrication of organic electronicdevices, plastics materials being preferred, examples including alkydresins, allyl esters, benzocyclobutenes, butadiene-styrene, cellulose,cellulose acetate, epoxide, epoxy polymers, ethylene-chlorotrifluoroethylene, ethylene-tetra-fluoroethylene, fibre glass enhanced plastic,fluorocarbon polymers, hexafluoropropylenevinylidenefluoride copolymer,high density poly-ethylene, parylene, polyamide, polyimide, polyaramid,polydimethylsiloxane, polyethersulphone, polyethylene,polyethylenenaphthalate, polyethyleneterephthalate, polyketone,polymethylmethacrylate, polypropylene, polystyrene, polysulphone,polytetrafluoroethylene, polyurethanes, polyvinylchloride, siliconerubbers, silicones. Preferred substrate materials arepolyethyleneterephthalate, polyimide, and polyethylenenapthalate. Thesubstrate may be any plastic material, metal or glass coated with theabove materials. The substrate should preferably be homogenous to ensuregood pattern definition. The substrate may also be uniformly pre-alignedby extruding, stretching, rubbing or by photochemical techniques toinduce the orientation of the organic semiconductor in order to enhancecarrier mobility.

Application of the invention in the manufacture of OFETs is nowdescribed in more detail.

Device layers in the case of OFETs (also referred to as OFET layers) maybe independently selected from a conductor, a dopant, an insulator or anOSC.

Where the OFET layer is a conductor it may be inorganic or organic or acomposite of the two. The conductor may provide an electrode for an OFETor provide an interconnect between the OFET and other elements. Theconductor may also act as part of a passive circuit element in an OFETcircuit, for example a capacitor, conductor or antenna for a radiofrequency tag (RF-tag). Conductors that are deposited by liquid coatingto enable ambient processing are preferred. Examples are polyaniline,polypyrrole, PEDOT or doped conjugated polymer. Further examples aredispersions or pastes of graphite or particles of metal such as Au, Ag,Cu, Al, Ni or their mixtures. Organometallic precursors may also be useddeposited from a liquid phase. Conductors are preferably spray-, dip-,web- or spin-coated or deposited by any liquid coating technique. Anyliquid medium may be employed as long as it does not dissolve thelift-off ink. If required, conductive layers may be deposited fromvapour phase.

Combining printing with a lift-off process a true high conductivitymetal layer can be patterned by printing into an OFET. Metal OFETelectrodes have previously been patterned only by lithography or shadowmask evaporation. OFETs are useful for radio frequency tag circuits,which require a high conductivity antenna. The fabrication of theantenna is particularly problematic as the polyaniline (PANI) orpoly(3,4-ethylenedioxy)thiophene-2,5-diyl (PEDOT) tracks used forprinted OFETs are sufficiently conductive for source, drain and gateelectrodes but not conductive enough for an efficient antenna. Thepresent approach allows the manufacture of devices where patterning iscarried out by printing for both soluble and insoluble components usingthe same process.

When a p-channel OFET is doped to increase the hole density in a certainregion of the device, acceptor-like compounds may be used as dopants. Asuitable dopant may be any acceptor-like compound, e.g.tetracyanoethylene; 3-nitrobiphenyl; 2,6-dimethyl-p-benzoquinone;2,3,5,6-tetrafluoro-p-benzoquinone (TFBQ);2,3,5,6-tetrachloro-p-benzoquinone (TCBQ); o-chloranil; p-chloranil;2,4,7-trinitrofluorenone; pyromellitic dianhydride; fullerenes;1(benzamido)-4-nitronaphthalene; tetracyanoquinodimethane (TCNQ);2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (4FTCNQ);diphenoquinones; bathophenanthrolines; and organic acids. Since n-typeorganic semiconductors are typically acceptor-like in character, thedopant may be an n-type OSC. Dopants are preferably organic andpreferred dopants are TCNQ, 4FTCNQ and diphenoquinones.

When an n-channel OFET is doped to increase the electron density in acertain region of the device, suitable dopants are donor-like. Anydonor-like compounds can be used, e.g. dibenzothiophene, phenothiazine,tetramethyl-p-phenylenediamine bis(ethylene-dithio)-tetrathiafulvalene.The dopant may also be a p-type OSC material, i.e. one which due to itsdonor character forms a complex with the n-type OSC. Organic bases mayalso be used.

Where the OFET layer is an insulator it may be inorganic or organic or acomposite of the two. It is preferred that the insulator is solutioncoated which allows ambient processing. When the insulator is beingpatterned, it may perform the function of interlayer insulation or actas gate insulator for an OFET. The insulator may be any organic polymeror polymer precursor, optionally containing inorganic particles. Theinsulator can be spray-, dip-, web- or spin coated or deposited by anyliquid coating technique. Any liquid medium may be employed as long asit does not dissolve the lift-off ink. Preferred gate insulators arethose that provide a low permittivity interface to the semiconductor.This can be achieved by a single or multilayer insulator structure asdescribed in our co-pending patent application PCT/GB01/05145.

Where the OFET layer is an OSC, it may be an n- or p-type OSC, which canbe deposited by vacuum or vapour deposition, or from solution and ispreferably deposited from a solution. Preferred OSCs have a FET mobilityof greater than 10⁻⁵ cm²V⁻¹s⁻¹.

The OSC is used as the active channel material in an OFET or a layerelement of an organic rectifying diode. OSCs that are deposited byliquid coating to allow ambient processing are preferred. OSCs arepreferably spray-, dip-, web- or spin-coated or deposited by any liquidcoating technique. Ink-jet deposition is also suitable. Any liquidmedium may be employed as long as it does not dissolve the lift-off inkwhen the OSC is patterned. The OSC may optionally be vapour deposited.

The OSC may be any conjugated aromatic molecule containing at leastthree aromatic rings. The OSCs preferably contain 5, 6 or 7 memberedaromatic rings, and more preferably contain 5 or 6 membered aromaticrings.

Each of the aromatic rings may optionally contain one or more heteroatoms selected from Se, Te, P, Si, B, As, N, O or S, preferably from N,O or S.

The aromatic rings may be optionally substituted with alkyl, alkoxy,polyalkoxy, thioalkyl, acyl, aryl or substituted aryl groups, halogen,particularly fluorine, cyano, nitro or an optionally substitutedsecondary or tertiary alkylamine or arylamine represented by —N(R³)(R⁴),where R³ and R⁴ each independently is H, optionally substituted alkyl,optionally substituted aryl, alkoxy or polyalkoxy groups. Where R³ andR⁴ is alkyl or aryl these may be optionally fluorinated.

The rings may be optionally fused or may be linked with a conjugatedlinking group such as —C(T₁)═C(T₂)—, —C≡C—, —N(R′)—, —N═N—, (R′)═N—,—N═C(R′)—. T₁ and T₂ each independently represent H, Cl, F, —C≡N orlower alkyl groups particularly C₁₋₄ alkyl groups; R′ represents H,optionally substituted alkyl or optionally substituted aryl. Where R′ isalkyl or aryl these may be optionally fluorinated.

Other OSC materials that can be used in this invention includecompounds, oligomers and derivatives of compounds of the following:

conjugated hydrocarbon polymers such as polyacene, polyphenylene,poly(phenylene vinylene), polyfluorene including oligomers of thoseconjugated hydrocarbon polymers; condensed aromatic hydrocarbons such astetracene, chrysene, pentacene, pyrene, perylene, coronene; oligomericpara substituted phenylenes such as p-quaterphenyl (p-4P),p-quinquephenyl (p-5P), p-sexiphenyl (p-6P); conjugated heterocyclicpolymers such as poly(3-substituted thiophene), poly(3,4-bisubstitutedthiophene), polybenzothiophene, polyisothianapthene, poly(N-substitutedpyrrole), poly(3-substituted pyrrole), poly(3,4-bisubstituted pyrrole),polyfuran, polypyridine, poly-1,3,4-oxadiazoles, polyisothianaphthene,poly(N-substituted aniline), poly(2-substituted aniline),poly(3-substituted aniline), poly(2,3-bisubstituted aniline),polyazulene, polypyrene; pyrazoline compounds; polyselenophene;polybenzofuran; polyindole; polypyridazine; benzidine compounds;stilbene compounds; triazines; substituted metallo- or metal-freeporphines, phthalocyanines, fluorophthalocyanines, naphthalocyanines orfluoronaphthalocyanines; C₆₀ and C₇₀ fullerenes; N,N′-dialkyl,substituted dialkyl, diaryl or substituteddiaryl-1,4,5,8-naphthalenetetracarboxylic diimide and fluoroderivatives; N,N-dialkyl, substituted dialkyl, diaryl or substituteddiaryl 3,4,9,10-perylenetetracarboxylicdiimide; bathophenanthroline;diphenoquinones; 1,3,4-oxadiazoles;11,11,12,12-tetracyanonaptho-2,6-quinodimethane;α,α′-bis(dithieno[3,2-b2′,3′-d]thiophene); 2,8-dialkyl, substituteddialkyl, diaryl or substituted diaryl anthradithiophene;2,2′-bibenzo[1,2-b:4,5-b′]dithiophene. Preferred compounds are thosefrom the above list and derivatives thereof which are soluble.

A preferred class of OSCs has repeat units of Formula 1:

where each Y¹ is independently selected from P, S, As, N and Se andpreferably polyarylamines, where Y¹ is N; Ar¹ and Ar² are aromaticgroups and Ar³ is present only if Y¹ is N, P, or As in which case it toois an aromatic group. Ar¹, Ar² and Ar³ may be the same or different andrepresent, independently if in different repeat units, a multivalent(preferably bivalent) aromatic group (preferably mononuclear butoptionally polynuclear) optionally substituted by at least oneoptionally substituted C₁₋₄₀ carbyl-derived groups and/or at least oneother optional substituent, and Ar³ represents, independently if indifferent repeat units, a mono or multivalent (preferably bivalent)aromatic group (preferably mononuclear but optionally polynuclear)optionally substituted by at least one: optionally substituted C₁₋₄₀carbyl-derived group and/or at least one other optional substituent;where at least one terminal group is attached in the polymer to the Ar¹,Ar² and optionally Ar³ groups located at the end of the polymer chains,so as to cap the polymer chains and prevent further polymer growth, andat least one terminal group is derived from at least one end cappingreagent used in the polymerisation to form said polymeric material tocontrol the molecular weight thereof.

WO 99/32537 and WO 00/78843 describe certain novel oligomers andpolymers, which have repeat units of Formula 1. The disclosures in theseapplications, particularly the novel oligomers and polymers areincorporated herein by reference, as these materials are particularlyuseful as OSCs in the present invention.

The number average degree of polymerisation is denoted by n and thenumber of the repeat units of Formula 1 which may be present permolecule in the invention may be from 2 to 1,000, preferably from 3 to100 and more preferably from 3 to 20. The polymer may comprise a mixtureof different polymeric species of varying chain lengths and with adistribution of molecular weights (polydisperse) or consist of moleculesof a single molecular weight (monodisperse).

The preferred polymeric materials are obtainable by polymerisationcontrolled by the addition of at least one end capping reagent in anamount sufficient to reduce substantially further growth of the polymerchain.

The asterisks extending from Ar¹ and Ar² in Formula 1 are intended toindicate that these groups may be multivalent (including divalent asshown in Formula 1).

The invention also includes polymers further substituted with, onaverage, more than one aryl moiety which is further substituted with amoiety capable of chain extension or cross linking, for example byphotopolymerisation or by thermal polymerisation. Such moieties capableof chain extension are preferably hydroxy, glycidyl ether, acrylateester, epoxide, methacrylate ester, ethenyl, ethynyl, vinylbenzyloxy,maleimide, nadimide, trifluorovinyl ether, a cyclobutene bound toadjacent carbons on an aryl moiety or a trialkylsiloxy.

Other amine materials that may be useful in this invention aretetrakis(N,N′-aryl)biaryldiamines, bis(N,N′-[substituted]phenyl),bis(N,N′-phenyl)-1,1′-biphenyl-4,4′-diamines including 4-methyl,2,4-dimethyl and/or 3-methyl derivatives thereof,tetrakis(N,N′-aryl)biphenyl-4,4′diamine-1,1-cyclohexanes and theirderivatives, triphenylamine and its alkyl and aryl derivatives andpoly(N-phenyl-1,4-phenyleneamine),N-dibenzo[a,d]cycloheptene-5-ylidene-N′,N′-di-p-tolyl-benzene-1,4-diamine,(9,9-dimethyl-9H-fluorene-2-yl)-di-p-tolyl-amine and their derivatives.

Further polyarylamine materials which may be useful in this inventionhave the following formulae:

These molecules are prepared directly via a multi-stage chemicalsynthesis which produces each molecule in a chemically pure monodisperseform.

Related materials, which may also find use in this invention have alsobeen described in patent DE 3610649, EP 0669654-A (=U.S. Pat. No.5,681,664), EP 0765106-A, WO 97-33193, WO 98-06773, U.S. Pat. No.5,677,096 and U.S. Pat. No. 5,279,916.

Conjugated oligomeric and polymeric heterocyclic semiconductors maycomprise a repeat unit of an optionally substituted 5 membered ring andterminal groups A¹ and A² as shown in Formula 2:

in which X may be Se, Te or preferably O, S, or —N(R)— where Rrepresents H, optionally substituted alkyl or optionally substitutedaryl; R¹, R², A¹ and A² each independently may be H, alkyl, alkoxy,thioalkyl, acyl, aryl or substituted aryl, fluoro, cyano, nitro or anoptionally substituted secondary or tertiary alkylamine or arylaminerepresented by —N(R³)(R⁴), where R³ and R⁴ are as defined above. Thealkyl and aryl groups represented by R¹, R², R³, R⁴, A¹ and A² may beoptionally fluorinated. The number of recurring units in the conjugatedoligomer of Formula 2 is represented by an integer n, where n ispreferably from 2 to 14. Preferred oligomers have X═S, R¹ and R²═H. andA¹ and A²=optionally substituted C₁₋₁₂ alkyl groups, examples ofespecially preferred compounds are those in which A¹ and A²=n-hexyl andwhere n=4, alpha-omega-n-hexylquaterthienylene (alpha-omega 4T), n=5,alpha-omega-n-hexylpentathienylene (alpha-omega-5T), n=6,alpha-omega-n-hexylhexathienylene (alpha-omega-6T), n=7,alpha-omega-n-hexylheptathienylene (alpha-omega-7T), n=8,alpha-omega-n-hexyloctathienylene (alpha-omega-8T), and n=9,alpha-omega-n-hexylnonathienylene (alpha-omega-9T).

Oligomers containing a conjugated linking group may be represented byFormula 3:

in which X may be Se, Te, or preferably O, S, or —N(R)—, R is as definedabove; R¹, R², A¹ and A² as defined above for Formula 2. Linking group Lrepresents —C(T₁)═C(T₂)—, —C≡C—, —N(R′)—, —N═N—, (R′)═N—, —N═C(R′)— withT₁ and T₂ defined as above.

Polymers may have repeat units of the general Formula 4:

in which X, R¹ and R² are defined as above. The sub units may bepolymerised in such a way as to give a regio regular or a regio randompolymer comprising repeat units as shown in Formulae 4 to 6:

Polymers may have repeat units of the general Formula 7:

in which X is as defined above and the bridging group A is optionallyfluorinated C₁₋₆ alkyl, for examplepoly(3,4-ethylenedioxy)thiophene-2,5-diyl and poly(3,4-trimethyldioxy)thiophene-2,5-diyl.

Polymers may have repeat units of general Formula 8:

in which X, R¹ and R² are defined as above. Preferably one of R¹ or R²is an alkoxide of general formula C_(n)H_(2n+1)O— in which n is from 1to 20, and the other of R¹ or R² is H, poly(dodecyloxy-α,α′,-α,α″terthienyl) i.e. polyDOT₃.

Polymers may have repeat units of general Formula 9:

in which X is as defined above; R⁵ and R⁶ each independently is H, alkylor substituted alkyl, aryl or substituted aryl. The alkyl and arylgroups may be optionally fluorinated.

Polymers may have repeat units of general Formula 10:

in which R⁷ and R⁸ each independently is optionally substitutedC₁₋₂₀-hydrocarbyl, C₄₋₁₆-hydrocarbyl carbonyloxy, C₄₋₁₆aryl(trialkylsiloxy) or both R⁷ and R⁸ may form with the 9-carbon on thefluorene ring a C₅₋₂₀ ring structure or a C₄₋₂₀ ring structurecontaining one or more heteroatoms selected from S, N or O.

Polymers may have repeat units of general Formula 11:

wherein R⁹ is C₁₋₂₀ hydrocarbyl optionally substituted with di(C₁₋₂₀alkyl)amino, C₁₋₂₀ hydrocarbyloxy or C₁₋₂₀ hydrocarbyl ortri(C₁₋₁₀alkyl)siloxy.

Copolymers comprising repeat units as described above and other repeatunits comprising two or more of the repeat units could be used.Copolymers preferably comprise one or more repeat units of Formula 10 orFormula 11 and Formula 1. In a further preference copolymers compriseone or more repeat units of Formula 1 and one or more repeat units of atleast one of Formulae 2 to 9.

In co-pending patent application PCT/GB01/05145 we describe OFETs madeusing solution coated compositions of an organic semiconductor and abinder polymer. The semiconductor compositions in that document are alsoincorporated herein for use in the present invention.

Where the semiconductor is a p type semiconductor this is preferably apolydisperse polyarylamine, mixtures of monodisperse polyarylamines,fluoro arylamine co-polymers, or cross-linkable arylamines.

Where the semiconductor is an n type semiconductor this is preferably afluorophthalocyanine, or a substituteddiaryl-1,4,5,8-naphthalenetetracarboxylic diimide and its oligomers.

The semiconducting channel may also be a composite of two or more of thesame types of semiconductors. Furthermore, a p type channel materialmay, for example be mixed with n-type materials for the effect of dopingthe layer. Multilayer semiconductor layers may also be used. For examplethe semiconductor may be intrinsic near the insulator interface and ahighly doped region can additionally be coated next to the intrinsiclayer. The invention provides the means to pattern multilayers ofsemiconductors in a single step with good edge accuracy. It alsoprovides the means to pattern locally doped regions in an OFET circuit.

Application of the invention in the manufacture of OLEDs is nowdescribed in more detail.

The OLED comprises at least an anode (electron blocking layer or holeinjection electrode), a cathode (hole blocking layer or electroninjection electrode) and an electroluminiscent layer. The OLEDoptionally comprises other layers such as an a hole injection layer(s),a hole transport layer(s), an electron injection layer(s), an electrontransport layer(s), a dopant, an insulator(s), a conductor orinterconnect. All or any of the aforementioned layers constitute devicelayers as referred to herein which can be patterned according to thepresent invention.

The electroluminescent layer is made up of substantially organic ororganometallic electroluminescent materials. Suitable materials includeorganic photo- or electroluminescent, fluorescent and phosphorescentcompounds of low or high molecular weight. Suitable low molecular weightcompounds include, but are not limited to, substituted 9,9′spirobifluorenes (EP 0676461), Alq3 (an aluminum complex formed bycoordination of three molecules of hydroxyquinoline with an aluminumatom), lanthanide complexes such as those of europium and ytterbium (WO9858037), triplet emitters such as Ir[2-PhPy]₃. Suitable high molecularweight materials include polymers preferably those having substantiallyconjugated backbone (main chain), such as polythiophenes,polyphenylenes, polythiophenevinylenes, polyphenylenevinylenes,polyalkylfluorenes. In the present invention the term polymer includeshomopolymer, copolymer, terpolymer and higher homologous as well asoligomers. Examples of such mateials are given in U.S. Pat. No.5,708,130, WO97/39082, WO96/10598.

The electroluminescent layer preferably has an average thickness of from50 to 200 nm, more preferably from 60 nm to 150 nm.

The electron blocking layer (hole injection electrode) is suitably madeof a metal or an alloy having a high work function such as Au, Pt, Ag.Preferably, a more transparent electron blocking layer (hole injectionelectrode) material such as an indium tin oxide (ITO) is used.Conductive polymers such as polyaniline (PANI) and apoly-3,4-ethylenedioxythiophene (PEDOT) are also suitable transparenthole-injection electrodes. Preferably, the electron blocking layer (holeinjection electrode) has a thickness of from 50 to 300 nm.

Hole-injecting and hole-transporting layer materials include solublephthalocyanine compounds, triarylamine compounds, electroconductivepolymers, perylene compounds, and europium complexes.

Electron-injecting and electron-transporting layer materials includeAlq3, azomethine zinc complexes, and distyrylbiphenyl derivatives. Theseare however not exhaustive.

The electron injection electrode is preferably made of a metal or analloy having a low work function, such as Yb, Ca, Al, Mg:Ag, Li:Al, Baor is a laminate of different layers such as Ba/Al or Ba/Ag electrode.

Dopants may be compounds such as3-(2-Benzothiazolyl)-7-diethylaminocoumarin (Coumarin 6), europiumcomplexes, ruthenium complexes, Rhodamine salts, platinum complexes,iridium complexes and Nile red although this list is not exhaustive.

Insulators used in the invention for OLEDs may be inorganic or organicor a composite of the two. It is preferred that the insulator issolution coated enabling ambient processing. When the insulator is beingpatterned, it may perform the function of a blocking layer between OLEDmaterials. The insulator may be any organic polymer or polymerprecursor, optionally containing inorganic particles. The insulator canbe spray-, dip-, web- or spin coated or deposited by any liquid coatingtechnique. Any liquid carrier may be employed as long as it does notdissolve the liftoff ink.

Specifically, the invention also provides methods of forming an OFET andan OLED respectively.

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 a) shows a profile of material printed directly on a flatsubstrate;

FIG. 1 b) shows the profile of material printed into a pre-patternedwell on a flat substrate;

FIG. 2 shows benefits of indirect printing to create a uniform patternin the plane of the substrate;

FIG. 3 shows indirect printing by ink-jet, stamping and microcontactprinting;

FIG. 4 shows an example of a vertical OFET device by indirect printingand self-alignment;

FIG. 5 shows the combination of printing and lift-off used to createvertical OFET structures by patterning simultaneously the verticallyaligned source, gate and drain electrodes;

FIG. 6 shows a print process for horizontal self alignment in the planeof the substrate;

FIG. 7 shows indirect patterning used to achieve vertical self-alignmentto print source and drain electrodes covered with a layer of dopant orinjection layer or blocking layer;

FIG. 8 shows an example of a creating a vertically aligned via;

FIG. 9 shows the lift-off method may be used to pattern self-assembledmonolayers (SAMs) aligned with a pattern underneath;

FIG. 10 shows an FET structure (described in Example 1) with a TiOPccharge injection layer deposited on top of Au/Ti layers produced by theink-jet lift-off method;

FIG. 11 shows a device structure (described in Example 2) comprisingpatterned multilayer of Pt/Pd and Poly(N-Methylpyrrole);

FIG. 12 shows schematically the deposition and lift-off steps in thepatterning procedure described in Example 2; and

FIG. 13 shows a SEM image of a vertically aligned trilayer structurepatterned onto a polyester film (described in Example 3).

FIG. 14 shows the structure of triarylamine OSC used in Example 3.

In the present invention indirect patterning is utilised to achieveself-aligned structures.

As FIG. 1 illustrates, direct printing often results in layers that arenot uniform in thickness whether the material to be patterned 11 isprinted on a flat substrate 12 or into a predefined well 13.

In an indirect process, a negative pattern is printed with a separate,specially selected ink 21 (i.e. the lift-off ink) as shown in FIG. 2.The layer to be patterned 23 can be deposited over the whole area of thesubstrate 22. Areas of the layer 23 are then removed together with theprint pattern 21. One advantage of this is that the layer material 23(e.g. organic semiconductor) does not have to be formulated into aprinting ink. There is a much higher degree of freedom in designing andformulating an inert ink 21, since additives can be used and theviscosity and flow properties are easier to control. A further advantageof an indirect process is that good uniformity and edge definition oflayer 23 is achieved as shown in FIG. 2 c. This is significantly betterthan that shown in FIG. 1, layer 11. The profile of the depositedmaterial 11 will normally be thinner at its edges when deposited on aflat substrate 12 as shown in FIG. 1 a, or a prepatterned well 13 asshown in FIG. 1 b.

The ink pattern may, for example be applied by ink-jet, stamping ormicrocontact printing as illustrated in FIG. 3 by 31 a, 31 b, 31 c. Thematerial to be patterned is then coated onto this layer uniformly byspin-(33 a), spray-(33 b), dip-(33 c), web coating, evaporation orink-jet printing. Subsequently the ink and the unwanted material can beremoved using a lift-off process. Unfortunately the feature size islimited to the resolution of the print technique used, which is notalways satisfactory.

However, with the present invention, it is possible to obtain deviceswith dimensions of the active area (e.g. FET channel length) smallerthan the print resolution by certain processes using indirect printing.It is also possible to manufacture improved devices with excellentalignment between layers despite using low resolution print techniques.

In one embodiment, a layer printed at low resolution is used to create avertical transistor by a process shown in FIG. 4. Here, an inert,lift-off, ink pattern 42 is printed on substrate 41. Onto this pattern aconductor layer 43, a semiconductor layer 44 and a second conductorlayer 45 are deposited. Following removal of the ink pattern 42 togetherwith portions of the layers above, a vertically aligned pair ofelectrodes 43 and 45 are formed with the semiconductor between them asshown in FIG. 4 c. This structure is then coated with an insulator layer46, preferably from the liquid phase. Finally a gate electrode 47 ispatterned using any of the aforementioned techniques. In thisembodiment, an OFET may be created by ink-jet printing with source anddrain electrodes separated by a very small distance set by the thicknessof the semiconductor layer 44. Such short OFET channels would otherwisebe difficult to print with direct ink-jet printing described in theprior art.

In another embodiment, a vertical OFET structure is formed by patterningsimultaneously the vertically aligned source, gate and drain electrodesas shown in FIG. 5. The ink pattern 52 is printed on substrate 51, forexample by ink-jet printing. Following this, conductive coatings 53, 55and 57 are deposited with intervening insulating layers 54 and 56 in thesequence shown. Layers 53-57 may be solution coated or optionallyevaporated. Following removal of the printed pattern 52 together withportions of other layers above, the mutilayer structure of 53-57 isformed as shown in FIG. 5 c. An organic semiconductor 58 is thendeposited, preferably by solution coating. In this vertical OFET,electrodes 53 and 57 are the source and drain and 55 acts as the gate.Thus a printed vertical OFET is realised with a channel length muchsmaller than otherwise would be possible by conventional direct ink-jetprinting. Note that the pattern of 42 or 52 in FIG. 4 or FIG. 5 can beproduced even by random droplets, each providing a vertical edge forminga transistor channel as shown in the top view of FIG. 5.d. Thereforeeven 1 f there is a scatter of the print alignment, vertical transistorswith channel lengths in the order of the film thickness e.g. 500 nm canstill be realised.

The above process may be used to pattern a multiplicity of overlying andalso neighbouring layers in a single step with good alignment. Otherprinting processes would have serious alignment or registration problemswhen printing one pattern over another. Using the present method makesit possible to print, for example, “self-aligned” OLED, OFET or otherorganic devices.

In another embodiment, the present invention provides the means toperform doping in a controlled, patterned manner. Often it is desirableto dope locally a semiconductor near contact areas to improve carrierinjection. However, ensuring that the dopant is only near the contact isnot easy in a printing process. In the process shown in FIG. 6 sourceand drain electrodes are patterned by printing with a layer of dopant onthem. An inert, lift-off ink layer is used to define these electrodesand the doping pattern simultaneously. The ink 62 is printed onto thesubstrate 61. Next, the electrode material 63 is deposited, for exampleby spin coating, spraying or evaporation, followed by a thin, 1-20 nmlayer of an organic dopant 64. A lift-off step then removes unwantedelectrode areas of the electrode material 63 and dopant 64 together withthe ink 62 in one step. The resulting electrodes have dopant richsurfaces as shown in FIG. 6 c. Subsequently the organic semiconductor 65and gate dielectric 66 are applied, preferably from solution phase.Finally gate electrode 67 is deposited. Local doping in this manner mayalso be preformed in an OFET channel, or at anode or cathode regions ofOLEDs. Analogously, electron or hole blocking layers can also be alignedwith contact areas.

In a further embodiment two different layers are horizontally alignedusing a negative print process. FIG. 7 a illustrates a substrate 71 witha printed inert ink layer 73. A layer of a semiconductor 72 is coated onthe printed pattern. Following this, a further sacrificial layer ofanother inert material 74 is coated deposited above. By affecting theliftoff of pattern 73, unwanted areas of both 72 and 74 are removed asshown in FIG. 7 b. Now another semiconductor layer 75 is coated onto thestructure as shown in FIG. 7 c. A second liftoff step (using a differentsolvent or removal method) removes the remaining portion of inertmaterial 74 together with the areas of 75 above it. Thus, a high degreeof alignment between the edges of the semicondcutor layers 72 and 75 isachieved despite the original printing method having low resolution.Note that semiconductors, insulators, conductors, doping materials orblocking layers or their combination may also be patterned this way toobtain horizontal self-alignment. For example, colour pixels of smallmolecule OLED displays can be formed in close proximity.

The printing technique described herein can further be used to createaligned vias in multilayer OFET circuits. For example a single ink-jetdot printed below an interlayer insulator can be used to create aninterconnect opening by lift-off. FIG. 8 provides an example of a gateelectrode connected to a source of an OFET. Source and drain electrodes82 are present on substrate 81. The lift-off ink 83 is printed at thedesired place for the via on electrode 82. Following the deposition ofthe semiconductor 84 and the insulator 85 an opening is created throughthem by lift-off as shown in FIG. 8 d. Finally a conductive layer 86 isdeposited, preferably from solution phase which makes connection to 82.The lift-off pattern for vias may be produced by any printing technique.The insulator itself may be spin or spray coated which ensures itsuniformity even for very thin layers. Defect-free direct printing ofuniform, thin layers of insulators by ink-jet or screen printing isdifficult.

The invention may be used to pattern aligned self-assembled monolayers(SAMs). SAMs deposited for example over the channel area of an OFET areoften required to improve the orientation of the semiconductor. SAMs mayalso improve carrier injection into organic materials. However, it wouldbe desirable to ensure that a SAM layer is only deposited over a certainpart of the device. In one embodiment of the present invention anorganic device element is patterned with a SAM layer on it withexcellent alignment. As FIG. 9 illustrates, an inert ink is printed on asurface 91. This is in turn coated with a device layer 96 which isfurther reacted with molecules that form SAMs. The device layer 96 maybe for example indium tin oxide (ITO), a metal (such as gold, silver, oraluminium) or an organic conductor. Suitable SAM molecules 92 can beanything that will covalently (or otherwise) bond with the surface andprovide it with resistance against dissolution in an etch solution. TheSAM may alternatively have a terminal functionality that promoteswetting (such as OH, COOH) or de-wetting (such as CH₃, CF₃) in order topattern a second polymer layer that could serve as an etch mask. Thesurface is reacted with the SAM either by immersing in a solution of themolecules, placing in a vapour stream of the molecules, or bringing intocontact with a planar elastomeric stamp 93 such as polydimethysiloxane(PDMS), which has been coated with the molecules from solution. Once theSAM has formed, the ink, portions of device layer 96 and loosely boundmolecules are lifted-off by immersing in a liquid medium to produce thepatterned device layer 96 with the SAM 92 aligned to it as shown in FIG.9 c.

In certain cases it may be desirable to pattern a SAM on OFET electrodesto change the electrical properties, such as modifying the work functionof the electrodes. Layer 96 may be a metal, in which case a patternedanode or cathode or source/drain electrodes are obtained with alignedSAM on them.

The indirect print processes for alignment described here may also beused as a complementary tool together with direct patterning techniquesfor OFETs e.g. with those described in WO 01/46987 (Plastic Logic,2001). A particularly advantageous feature of the present method is thatthe patterning of a number of materials/layers with the same ink ispossible. Using the same ink for many different constituent layers ishighly advantageous as it greatly simplifies the process. A furtheradvantage of the present indirect process is that the uniformity of thethickness of the printed pattern is not important—unlike when using adirect printing approach. The printed lift-off pattern will provideexcellent edge definition even when the printed area is thin at itsedges.

EXAMPLES Example 1 Patterning an Injection Layer by the SimultaneousLift-Off of a Stack of Ti, Au, and TiOPc Charge Generation Layer

A negative pattern of the source and drain electrodes of a FET wasprinted onto a polyester film using an Epson C60 ink-jet printer. Thesample was then placed in an Edwards Auto 306 evaporator and 5 nm of Tifollowed by 27 nm Au was coated onto the ink. A dispersion of Titanyloxyphthalocynine TiOPc in butyl acetate was created by shaking 0.46 gTiOPc & 10 g of 2% w/v solution of polyvinyl butyryl with 27 g of 3 mmglass beads in a “red devil” for 1 hour. The TiOPc dispersion was spincoated onto the Au/Ti/ink pattern at 1000 rpm for 20 s and then wasimmediately sonicated for 20 s in methanol to remove the ink & theovercoated layers in these areas. An image of the coated electrodes isshown in FIG. 10 (NB. gap between lines ˜0.1 mm).

As can be seen the TiOPc only coats the electrodes and not thesubstrate, hence this type of patterning can be used to depositinjection layers (materials that improve charge injection into organicsemiconductors) on top of metals that would not normally form ohmiccontacts with OSCs in OFET applications. A uniform layer of the TiOPcthroughout the OFET channel would most likely increase the off currentin the device resulting in a poor on/off ratio hence is not desirable.

Example 2 Preparation of Patterned Poly(N-Methylpyrrole) Films on Pt/Pdby “Lift-Off” Procedure

Method of Preparation of Poly(N-Methylpyrrole) Films

A 25 nm layer of sputter-coated platinum/palladium was deposited upon apattern produced by ink-jet printing. This was then immersed in asolution of iron (III) chloride hexahydrate (2.703 g, 0.0 mol) indistilled water (100 mL), which had been previously outgassed withnitrogen. To this solution was added N-methylpyrrole (1 mL, 0.01 mol)and the mixture was maintained at between 25 and 27° C. internaltemperature.

Polymerisation of the N-methylpyrrole was immediately initiated by thecatalyst solution. The reaction was continued for the required“deposition” time, usually between 30 and 120 mins, before the polymercoated substrate was removed from the reaction mixture, washed withdistilled water and ethanol and blown dry with a flow of nitrogen gas.The polymer produced by the above methodology was black in colour andinsoluble in all organic solvents.

The polymer-coated metallised substrate was placed in a small volume(approximately 100 mL) of methanol and lowered into an ultrasonic bathfor 2 minutes. The substrate was then removed and washed with methanoland the process was repeated before the substrate was dried with astream of compressed air followed by drying in an oven at 100° C. for 5minutes. At this stage each patterned device was examined by opticalmicroscopy to determine the extent of lift-off and an image of a typicaldevice is shown below (see FIG. 11). It was noted that lift-off wasgenerally very successful in cases where a thin polymer film had beendeposited (e.g. 30 mins deposition time). The process is illustratedschematically in FIG. 12.

The deposition of such a layer of polymer can improve the chargeinjection from a metal to an organic semiconductor. In this case thepolymer is a doped and therefore would cause an increase in the offcurrent of an OFET if it were deposited uniformly. Hence by patterningit using the lift-off method the doping only occurs in the region of theelectrodes.

Example 3 Patterning Several Layers Simultaneously

Circular spots of ink were printed onto a polyester film using an EPSONC60 ink-jet printer. The film was placed in a sputter coater and 25 nmof Pt/Pd alloy was deposited onto the whole sample. Following this asolution of triarylamine OSC (M_(w)=4000) (FIG. 14) in toluene (5% wt)was deposited on the substrate and spun at 1000 rpm for 15 s to createan approximately 400 nm thick film. The sample was baked at 100° C. for20 min to evaporate the solvent. A further 25 nm layer of PVPd metal wassputtered over the whole sample. Lift-off of the tri-layer structure wasachieved by sonicating in methanol for ˜20 s followed by blow drying. ASEM image (FIG. 13) of the structure (taken at 45°) shows each layer ofthe structure (in FIG. 13 a=first electrode., b=OSC, c=second electrode)aligned at the edge of the lift-off area. This type of patterning may beused to form a vertical transistor with the two electrodes a) and c)serving as source and drain. A layer of insulator may be spun coatedover the step, and gate electrode may then be deposited on top. Thismethod allows fabrication of a transistor by printing and reduces thechannel length to very small dimensions, in this case to the thicknessof the semiconductor layer (i.e. ˜400 nm) without the need for highresolution photolithography.

1. A method of forming an organic electronic device, which methodcomprises the steps of: a) forming a negative image of a desired patternon a substrate or layer of the device with a lift-off ink; b) coating afirst device layer to be patterned on top of the negative image; c)coating one or more further device layers to be patterned on top of thefirst device layer to be patterned; and d) removing the lift-off ink andunwanted portions of the device layers above it, thereby leaving thedesired pattern of device layers; and e) wherein the organic electronicdevice comprises a vertical transistor.
 2. A method of forming anorganic electronic device as claimed in claim 1 wherein the lift-off inkis insoluble in the liquid medium used to deposit the device layers tobe patterned.
 3. A method of forming an organic electronic device asclaimed in claim 1 or 2 wherein the ink comprises a liquid medium whichdoes not dissolve the substrate or layer on which the ink is printed. 4.A method of forming an organic electronic device as claimed in claim 1wherein the lift-off ink is deposited on the substrate or layer by adirect printing technique selected from the following: ink-jet printing,screen printing, microcontact printing, stamping, soft lithography orelectrophotographic printing using a solid or liquid toner.
 5. A methodof forming an organic electronic device as claimed in claim 1 whereinthe deposited lift-off ink is thicker than the device layerssubsequently deposited onto it.
 6. A method of forming an organicelectronic device as claimed in claim 1 wherein the lift-off pattern isfrom 1 μm to 50 μm.
 7. A method of forming an organic electronic deviceas claimed in claim 1 wherein the ink is deposited by screen printingand the ink has a viscosity from 500 and 10,000 cP.
 8. A method offorming an organic electronic device as claimed in claim 1 wherein theink is deposited by ink-jet printing and the ink viscosity is in therange from 3 to 40 cP.
 9. A method of forming an organic electronicdevice as claimed in claim 1 wherein the ink has a surface tension inthe range of 20-60 dynes/cm.
 10. A method of forming an organicelectronic device as claimed in claim 1 wherein the surface tension ofthe ink relative to the substrate is in the range 40-80 deg.
 11. Amethod of forming an organic electronic device as claimed in claim 1wherein the lift-off ink contains from 50% to 99.8% liquid medium, byweight.
 12. A method of forming an organic electronic device as claimedin claim 1 wherein the lift-off ink further comprises a colorant, apolymeric binder or one or more functional additives.
 13. A method offorming an organic electronic device as claimed in claim 1 wherein thelift-off ink further comprises a cross-linking agent to permitcross-linking of the printed ink.
 14. A method of forming an organicelectronic device as claimed in claim 1 wherein partial shrinkage ormicro-cracks are induced to allow a lift-off medium to penetrate the inkat the pattern edges or through its surface to aid the lift-off step(d).
 15. A method of forming an organic electronic device as claimed inclaim 1 wherein wetting of the ink is effected by a surface treatment ofthe substrate.
 16. A method of forming an organic electronic device asclaimed in claim 1 wherein the device layers to be patterned are eachindependently applied by solution-, spin-, spray-, dip-, web-, die- orevaporation coating.
 17. A method of forming an organic electronicdevice as claimed in claim 1 wherein the device layer to be patterned isapplied by electroless deposition, ink-jet printing, screen printing,microcontact printing, stamping or soft lithography.
 18. A method offorming an organic electronic device as claimed in claim 1 wherein thethickness of each device layer or multiplicity of layers is from 1 nm to1 μm.
 19. A method of forming an organic electronic device as claimed inclaim 1 wherein the lift-off step (d) includes dissolving the lift-offink using a lift-off liquid medium.
 20. A method of forming an organicelectronic device as claimed in claim 19 wherein the lift-off liquidmedium dissolves little or none of the device layer to be patterned. 21.A method of forming an organic electronic device as claimed in claim 19or 20 wherein the lift-off step (d) further includes ultrasonicagitation, stirring, spraying liquid medium and/or heating.
 22. A methodof forming an organic electronic device as claimed in claim 1 whereinthe device is an OFET and the device layers are each independentlyselected from a conductor, a dopant, an insulator or an organicsemiconductor (OSC).
 23. A method of forming an organic electronicdevice as claimed in claim 22 wherein the device layers include aconductor that is deposited by liquid coating.
 24. A method of formingan organic electronic device as claimed in claim 23 wherein theconductor is selected from the group comprising polyaniline,polypyrrole, PEDOT, doped conjugated polymer, or dispersions or pastesof graphite or particles of metal including Au, Ag, Cu, Al, Ni or theirmixtures.
 25. A method of forming an organic electronic device asclaimed in claim 22 wherein the device layers include an OSC comprisinga polymer or oligomer including monomers of triarylamine, fluorene, orthiophene, including substituted forms thereof.
 26. A method of formingan organic electronic device as claimed in claim 22 wherein the devicelayers include an OSC comprising pentacene or solution coated precursorpentacene.
 27. A method of forming an organic electronic device asclaimed in claim 22 wherein the device is a vertical OFET.
 28. A methodof forming an organic electronic device as claimed in claim 1 whereinthe device layers include an OSC, which is deposited from solution. 29.A method of forming an organic electronic device as claimed in claim 1wherein the step d) forms one or more via openings.
 30. A method offorming an organic electronic device as claimed in claim 1 wherein thedevice is an OLED and at least one of the device layers to be patternedis selected from an anode, a cathode or an electroluminiscent layer. 31.A method of forming an organic electronic device as claimed in claim 30wherein the electroluminescent layer comprises a substantially organicor organometallic electroluminescent material.
 32. A method of formingan organic electronic device as claimed in claim 31 wherein theelectroluminescent layer comprises a polymer or oligomer containingmonomers of thiophene, phenylene, thiophenevinylene, phenylenevinylene,or fluorene, including substituted forms thereof.
 33. A method offorming an organic electronic device as claimed in claim 1 wherein thedevice is an OLED and at least one of the device layers to be patternedis selected from a hole injecting layer, hole transporting layer,electron injecting layer, electron transporting layer or interconnect.34. A method of forming an organic electronic device as claimed in claim1 wherein the device is an OLED and at least one of the device layers tobe patterned is a dopant or an insulator.
 35. An organic electronicdevice obtainable by claim 1.